Method and apparatus for adjusting data timing by delaying clock signal

ABSTRACT

A circuit for adjusting a time when data is delivered to a data terminal with respect to an external clock signal includes a data passing circuit and a delay adjusting circuit. The delay adjusting circuit accepts a plurality of control signals each arranged to control passgates arranged in columns, with one column being controlled by a respective one of the control signals. A clock signal passes in parallel manner through a variety of delay gates, and each delay gate is coupled in series with one of the passgates. By selecting a path through desired passgates, one delay path is selected and the delay time added to the clock signal. This delayed clock signal is used to control the data passing circuit, which controls when data is output to the output terminals relative to the original clock signal. The control signals are created by selectively coupling or decoupling the control signals from a static voltage, and fuses or antifuses can be used to facilitate this coupling or decoupling.

TECHNICAL FIELD

This invention relates to clocked integrated circuits that deliver data,and more particularly to a method and apparatus for adjusting the timingof data presented to an output terminal relative to a clock signal.

BACKGROUND OF THE INVENTION

Clock signals are used by a wide variety of digital circuits to controlthe timing of various events occurring during the operation of thedigital circuits. For example, clock signals are used to designate whencommand signals, data signals, and other signals used in memory devicesand other computer components are valid and can thus be used to controlthe operation of the memory device or computer system. For instance, aclock signal can be used to develop sequential column addresses when anSDRAM is operating in burst mode.

Retrieving valid data from a clocked memory device at a specified timecan be difficult to coordinate. After a memory address is selected, thedata travels out of the selected memory cell, is amplified, passesthrough configuration circuitry (if the memory chip has multipleconfigurations) and passes through an output buffer before the data isread. Before the advent of synchronous memory circuits, data simplyappeared at an output terminal following a propagation delay after thedata was requested. In a synchronous memory circuit, data delivery issynchronized with a clock signal. Many circuits have been created tocoordinate data signals with clock signals, with varying degrees ofsuccess. Two of the problems to solve are determining how fast and withwhat regularity the data signal propagates through the chip circuitry.Because data output is often coordinated with a clock signal that isexternal to the memory chip, computer simulations of signal propagationwithin a chip are performed to align the external clock signal with thedata delay of the synchronous memory device. Static time delays are thendesigned into the memory circuit based on the simulation predictions.Because of production variations, improper assumptions, and otherfactors ultimately causing timing errors, the data does not alwaysarrive at the output terminal at the desired time. As computer clockspeeds increase, the window for providing valid data to the outputterminal closes, making it more difficult to ensure the correct deliverytime of data from the memory circuit.

An example of a circuit that provides data to a data pad at a specifictime relative to an external clock is shown in FIG. 1. An output circuit2 includes a memory array 5 that contains an array of individual memorycells (not shown). Once a particular memory cell is selected to be read,complementary signals corresponding to the contents of the memory celltravel to a pair of respective I/O and I/O* lines. The signals on theI/O and I/O* lines are sensed and amplified by a data sensing circuit10, which produces a DATA* signal at an output. An external clock signalis received at a clock circuit input 7 and passes through clockcircuitry 15 to become a CLKDOR* signal. The CLKDOR* signal may differfrom the external clock signal in a variety of ways, including phase,orientation, and duty cycle, however, their overall periodic cyclelength is the same. Oftentimes, to properly match timing of the dataarriving at the data pad with the external clock signal, a static delayis added within the clock circuitry 15.

The DATA* signal is presented to a passgate 20 and passed to an outputnode 21 when the signal CLKDOR* signal is HIGH and its complement froman inverter 17 is LOW. From the output node 21, the DATA* signal isinput to a NOR gate 30 along with a TRISTATE signal. An output from theNOR gate 30 leads to a passgate 24. When the CLKDOR* signal is LOW andits complement from the inverter 17 is HIGH, the output from the NORgate 30 passes through the passgate 24 and becomes the signal DQHI.Another NOR gate 32 combines the output of the NOR gate 30 with theTRISTATE signal. This output from the NOR gate 32 is presented to a pairof passgates 22, 26. The passgate 22 receives the signal from the NORgate 32 and, when the CLKDOR* signal is LOW and its complement from theinverter 17 is HIGH, feeds it back to the output node 21. The passgate26 passes the signal it receives from the NOR gate 32 as an outputsignal DQLO when the signal CLKDOR* is LOW and its complement from theinverter 17 is HIGH.

If the signal DQHI is HIGH, a pull-up circuit 36 raises a DQ pad 40 to aHIGH voltage. Conversely, if DQLO is HIGH, it activates a pull-downcircuit 38 to pull the DQ pad 40 to a ground voltage. The output circuit2 is designed so that the pull-up circuit 36 and the pull-down circuit38 cannot operate simultaneously. When neither the pull-up circuit 36nor the pull-down circuit 38 is active, the DQ pad 40 is neither pulledup to a HIGH voltage nor pulled down to ground, but instead remains in ahigh-impedance state.

The circuit operation of the data delivery circuit 2 will now beexplained. When the CLKDOR* signal is HIGH and the DATA* signal is HIGH,a HIGH signal passes to the output node 21. Assuming that the TRISTATEsignal is low to enable the NOR gates 30 and 32 so they act asinverters, when the CLKDOR* signal goes LOW, the passgate 22 couples theoutput of the NOR gate 32 to the input of the NOR gate 30, output node21. The NOR gates 30 and 32 then latch the HIGH at the output node 21 tothe output of the NOR gate 32. At the same time, a LOW is latched to theoutput of the NOR gate 30. The HIGH at the output of the NOR gate 32 iscoupled through the passgate 26 to the pull-down circuit 38. The HIGHsignal DQLO causes the pull-down circuit 38 to pull the DQ pad 40 toground. At the same time, the LOW signal at the output of the NOR gate30 passes through the passgate 24. The LOW DQHI signal does not activatethe pull-up circuit 36, as explained above. Alternatively, if the DATA*signal is LOW, a LOW signal is passed to the output node 21 when theCLKDOR* signal is HIGH. When the CLKDOR* signal drops LOW, the LOWsignal at the output node 21 is latched by the NOR gates 30 and 32, isfed back to the output node 21 through the passgate 22, and alsopropagates through the passgate 26 to make DQLO LOW. Concurrently, theLOW signal at the data output node 21 causes the NOR gate 30 to output aHIGH signal that passes through the passgate 24 to provide a HIGH DQHIsignal. The HIGH DQHI signal causes the pull-up circuit 36 to connectthe DQ pad 40 to a HIGH voltage. If the TRISTATE signal is HIGH, neitherDQHI nor DQLO will be HIGH regardless of the state of the DATA* signal.Thus, the DQ pad 40 floats in a high impedance state.

When a computer system is designed, specifications for signal timing aredetermined. Some of the signals and timings used in the design are shownin FIG. 2. One of the design specifications is an access time, T_(AC),used to designate a maximum time between a rising edge of an externalclock signal and when a valid data signal arrives at the DQ pad 40.Additionally, another specified time parameter is the output hold time,T_(OH), indicative of a minimum time for how long the data will be heldat the DQ pad 40 following a subsequent rising edge of the externalclock. For example, as illustrated in FIG. 2, a READ command signal isinput to a memory circuit sometime between a rising edge of a clockpulse CP0 and a clock pulse CP1. At a time CP1, the READ command islatched and read by the memory circuit, indicating data is to be readfrom a memory cell in a memory array. The data is read from the arrayand placed at the DQ pad 40 under the control of the CLKDOR* signal. Thespecification T_(AC) indicates a maximum time until the desired data isplaced on the DQ pad 40. The data is held at the DQ pad 40 for a time noless than the specification T_(OH), as measured from a subsequent clockpulse after the READ command is latched. As shown in FIG. 2, T_(AC1) isthe time measured from CP2 until Data₁, is stable on the DQ line.T_(AC2) is the time measured from CP3 until Data₂ is stable on the DQline, and so on. The time T_(AC1) will be nearly identical to the otheraccess times T_(AC2), T_(AC3), etc. under the same operating conditions.Also shown in FIG. 2, T_(OH1) is the time measured from the next clockpulse following when Data , appears on the DQ line, i.e., CP3, to thetime when Data₁ begins to transition off the DQ line. As above, themeasured hold times T_(OH2), TOH₃, etc. will be nearly identical to oneanother under similar operating conditions.

During the design phase of a memory chip, a designer determines how muchafter each clock pulse the CLKDOR* signal should fire. This delaydetermines when the data is made available on the DQ line relative tothe external clock signal. Typically, a delay value is chosen thatprovides a tolerance for both the T_(AC) and T_(OH) parameters. If theCLKDOR* signal fires too soon after the external clock signal, the chipwill easily pass the T_(AC) specification, but may fail the T_(OH)specification. If the CLKDOR* signal fires too late, the chip willeasily pass the T_(OH) specification but may fail the T_(AC)specification. These time compensations, by virtue of being fabricatedas part of the circuit, generally cannot be changed after manufacture ofan integrated circuit. When memory chips fail their timingspecifications, they are sold as lesser quality chips for a reducedprice, or even destroyed. Thus, there is an economic incentive tomaximize the number of chips that meet or exceed the timingspecifications. As a consequence of increasing computer speeds, thisalready small window for proper data timing is reducing. Because ofprocess variations, errors in design assumptions, the wide range oftemperatures and voltages in which the chips are warrantied to perform,and other factors, an increasing number of memory chips fail to meet theincreasingly stringent design specifications.

SUMMARY OF THE INVENTION

An adjustable data delay circuit comprises a clocked data passingcircuit that receives a clock signal and a data signal. An adjustabletime delay circuit is coupled to the clock signal for adjusting the timethe data is delivered to an output terminal relative to the clocksignal. The adjustable time delay circuit includes a plurality of delaygates, each individually selected by control signals. One path in thetime delay circuit that includes the desired delay gate is selected bythe control signals. The clock signal passing through the selected delaygate is then used to control the time when the data is delivered to theoutput terminal.

In one embodiment, the control signals are made by selectively couplinga pattern of control inputs to a reference voltage.

In another embodiment, the passgates are arranged in a plurality ofcolumns such that each column has a number of passgates that is aninteger power of 2.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a conventional clocked data deliverycircuit.

FIG. 2 is a timing diagram of various signals during the operation ofthe clocked data circuit of FIG. 1.

FIG. 3 is a schematic diagram of an adjustable clocked data circuitaccording to one embodiment of the present invention.

FIG. 4A is a schematic diagram of a delay adjusting circuit according toone embodiment of the present invention.

FIG. 4B is a chart showing how different delay times are selected usingone embodiment of the present invention.

FIG. 5A is a schematic diagram of a conventional adjustable impedancedevice.

FIG. 5B is a schematic diagram of another conventional adjustableimpedance device.

FIG. 6 is a block diagram of a synchronous dynamic random access memoryincluding adjustable time delivery circuit of FIG. 3.

FIG. 7 is a block diagram of a computer system including the randomaccess memory of FIG. 6.

DETAILED DESCRIPTION OF THE INVENTION

One embodiment of an adjustable time delay circuit 102 in accordancewith the invention is illustrated in the schematic diagram of FIG. 3.The adjustable delay circuit 102 includes some of the same components asthe output circuit 2, shown in FIG. 1. Identical components of theoutput circuit 2 and the adjustable time circuits 102 have been giventhe same reference numbers, and for the sake of brevity, identicalcomponents will not be described in further detail. The adjustable timedata circuit 102 includes a delay adjusting circuit 60 located betweenthe clock circuitry 15 and the control inputs to the passgates 20, 22,24, and 26. As later described, the delay adjustment circuit 60 can belocated in various places in the adjustable delay circuit 102, and isshown in this location of the adjustable delay circuit 102 forillustration.

As shown in FIG. 4A, the CLKDOR* signal is input to the delay adjustingcircuit 60 at an input terminal 62. From there it is split into fourpaths, passing through a delay circuit 70, having a delay of 1.0; adelay circuit 72, having a delay of 0; a delay circuit 74, having adelay of 0.5; and a delay circuit 76, having a delay of 1.5. The delaytimes 0, 0.5, 1.0, and 1.5 are an indication of relative measure and donot necessarily indicate a specific time period. These delay times areselected such that the delay that would have been designed into theoutput circuit 2 appears as a middle value of the range of delay valueseligible for selection. In this way, the data delivery of a memory chipcan be “accelerated” by selecting a delay time shorter than the built-indelay of the prior art circuit, or “decelerated” by selecting a delaytime longer than the built-in delay of the prior art circuit. The outputsignals from the delay circuits 70, 72, 74, and 76 are input to apassgate 80, a passgate 82, a passgate 84, and a passgate 86,respectively. The passgates 80, 82, 84, and 86 are controlled by acontrol signal A and its complement formed by passing A through aninverter 92. The signals from the passgates 80 and 82 combine as aninput to a passgate 88, and the signals from the passgates 84 and 86combine as an input to a passage 90. The outputs from the passgates 88and 90 connect at an output terminal 95, and form an output signal OUT.The passgates 88, and 90 are controlled by a control signal B and itscomplement formed by passing B through an inverter 94.

Referring back to FIG. 2, the benefits of having an adjustable CLKDOR*signal will be described. As previously stated, adding delay to theCLKDOR* signal in relation to the CLK signal allows the designer toprovide a tolerance for the T_(AC) and T_(OH) specifications. After thechip is produced, the T_(AC) and T_(OH) specifications, and others, aretested. If the chip does not pass all of the specifications, it cannotbe sold at the current market price for the highest quality chips. Byincluding an adjustable timing circuit within the memory chip, chipsthat do not meet the T_(AC) and T_(OH) specifications after manufacturemay be able to be adjusted in order to meet the specifications.

For example, the specifications may direct that T_(AC) can be no morethan 6 ns and T_(OH) cannot be less than 3 ns . Assume that T_(AC1)measured 4 ns and T_(OH1) measured 2.5 ns. The specification for T_(AC)is easily passed (the shorter the better), but the chip fails the T_(OH)specification because it does not hold the data for a long enough timeon the DQ lines. By adding a 1 ns delay to the time when the CLKDOR*signal fires, the chip can be brought within the specifications. TheT_(AC1) increases to 5 ns (still passing the 6 ns specification) and theT_(OH1) increases to hold the data valid on the DQ lines for 3.5 ns,passing the 3 ns specification.

The delay adjusting circuit 60 of FIG. 4A is controlled by controlsignals A and B. These control signals provide a HIGH or LOW signal tothe passgates depending on a state of a respective adjustable impedancecircuit 96. Two different kinds of adjustable impedance devices areshown, one in FIG. 5A and one in FIG. 5B. One type of adjustableimpedance circuit 96 is a circuit containing an antifuse 65, shown inFIG. 5A. The antifuse 65 is made from a pair of conducting plates 110and 112 separated by a dielectric material 115. Antifuses are devicessimilar to small capacitors. They have a natural and a blown state. Whenthe antifuse 65 is in a natural state, the dielectric material 115electrically insulates the pair of plates 110 and 112. Because thedielectric material 115 is intact, the node C is electrically insulatedfrom the ground voltage. To change the antifuse 65 to its blown state, ahigh electric field is passed across the dielectric material 115 byraising C_(gnd) to a programming voltage, for example, 10 volts, whileenabling a PROGRAM transistor. This is usually done after chipfabrication and packaging, but can be completed before packaging. Whenthe high electric field is placed across the dielectric material 115, itbreaks down and loses its insulative properties. This allows the plates110 and 112 to contact one another creating a relatively low resistivecontact. When blown, the antifuse 65 couples the node C to the nodeC_(gnd), that is normally held at the ground voltage, unless theantifuse is being programmed, as described above. To test the state ofthe antifuse 65 a Read* signal is strobed LOW. That connects node C tothe Vcc voltage. If the antifuse 65 is blown, the node C is quicklybrought down to ground. An inverter 50 causes a HIGH signal to be sentto a BLOWN output. The HIGH signal also keeps a HOLD transistor OFF.Conversely, if the antifuse 65 is in its natural state, node C will notbe pulled down to ground and BLOWN will carry a LOW signal. This lowsignal also enables the HOLD transistor, keeping node C at the voltageVcc.

The other adjustable impedance circuit 96, shown in FIG. 5B contains afuse 68. The fuse 68 also has a natural and a blown state. In itsnatural state, the fuse 68 couples a node D to the ground voltage. Thefuse 68 is blown by passing a high current through it, or by some othermeans such as cutting it with a laser, for example. When the fuse 68 isblown, the node D is disconnected from the ground voltage. As with theantifuse 65, the fuse 68 may be blown before or after packaging. Also asdescribed above, the adjustable impedance circuit 96 of FIG. 5B is readin a similar manner. The Read* signal strobes LOW raising a node D tothe Vcc voltage. If the fuse 68 is intact, node D is coupled to groundand BLOWN is LOW. This LOW signal passes through an inverter 52 to keepthe HOLD transistor OFF. If the fuse is blown, node D is charged to Vccand BLOWN is pulled HIGH.

Referring back to FIG. 4A, the adjustable impedance circuits 96 may beeither of the structures shown in FIGS. 5A or 5B. By coupling thesignals A and B to a voltage using antifuses 65 or fuses 68, themanufacturer can easily select the signals A and B to be either HIGH orLOW, as desired. Although described here as controlling only oneadjustable delay circuit 102, a single delay adjusting circuit 60 may beused to adjust any or all of the adjustable delay circuits within amemory chip, thereby controlling the data delivery time at any or all ofthe DQ pads on the memory chip.

The operation of the delay adjusting circuit 60 will now be described.In operation, one of the four delay times is selected through the statesof signals A and B, as shown in the chart in FIG. 4B. If A and B areeach connected to respective adjustable impedance circuits 96 that areBLOWN, both A and B will be HIGH, indicated as “1” in FIG. 4B. Thisplaces the passgates 82, 86, and 90 in a passing state. Because thepassgates 86 and 90 are passing, the signal CLKDOR* passes through thedelay gate 76 having a delay of 1.5, and through the passgates 86 and 90to the output terminal 95. The CLKDOR* signal also passes through thedelay gate 72, having no delay and through the passgate 82, but isblocked at the passgate 88, which is in a blocking state by virtue of aHIGH B signal and a LOW signal received from the inverter 94. Byselecting the states of the signals A and B (by selectively adjustingthe impedance circuits 96), it is easy to adjust the time delay of aclock signal input to the delay adjusting circuit 60. In one embodiment,the delay time selected by keeping the adjustable impedance circuits 96in their natural state will be the delay most likely to provide thegreatest tolerances for both T_(AC) and T_(OH). In FIG. 4A this desireddelay is 1.0. In this way, the majority of the memory chips will passthe T_(AC) and T_(OH) specifications without further adjustment, savinglabor and equipment costs. Only in the extraordinary case will the delayneed adjustment. Although shown here with only two columns of passgatescontrolled by the signals A and B, it is apparent that a greaterselection of delay times can be made available with the addition of morecontrol signals and more passgates, or that the passgates could have adifferent configuration. For instance, eight different delay times areefficiently selectable if three control signals are used, with threecolumns, one each containing two, four and eight passgates.

Although the delay adjusting circuit 60 is shown after the clockcircuitry 15, it can appear in many locations in a synchronized memorycircuit, some of which are illustrated in FIG. 3. For instance, thedelay adjusting circuit 60 can appear directly before the clockcircuitry 15. If the delay adjusting circuit 60 is placed after thepassgates 24 and 26, the delay adjusting circuit must be implemented inpairs because the data has two separate paths. Only one delay adjustingcircuit 60 is needed if it is located between an output terminal 37 andthe DQ pad 40. Of course, there are other locations where the delayadjusting circuit 60 could be placed, as long as it is between the clocksignal input 7 and the DQ pad 40.

A synchronous dynamic random access memory (SDRAM) 200 using theadjustable time delay circuit 102 of FIG. 3 is shown in FIG. 6. TheSDRAM 200 has a control logic circuit 202 receiving a clock signal CLKand a clock enable signal CKE. In the SDRAM 200, all operations arereferenced to a particular edge of an internal clock signal ICLK and adata read clock CLKDOR*, both generated from the clock signal CLK. Theedge of the ICLK signal that is used is typically the rising edge, whilethe data read operations are referenced to the falling edge of theCLKDOR*, as known in the art. The delay adjusting circuit 60 ispreferably included in the control logic 202 to adjust the timing of thedata read clock CLKDOR* relative to the clock signal CLK. In practice, avariety of internal clock signals may be generated from the clock signalCLK, and only some of them may have their timing controlled by the delayadjusting circuit 60. However, in the interest of brevity, only twointernal clock signals, ICLK and CLKDOR* are shown. The control circuit202 further includes a command decode circuit 204 receiving a number ofcommand signals on respective external terminals of the SDRAM 200. Thesecommand signals typically include a chip select signal {overscore (CS)},write enable signal {overscore (WE)}, column address strobe signal{overscore (CAS)}, and row address strobe signal {overscore (RAS)}.Specific combinations of these signals define particular data transfercommands of the SDRAM 200 such as ACTIVE, PRECHARGE, READ, and WRITE asknown in the art. An external circuit, such as a processor or memorycontroller generates these data transfer commands to read data from andto write data to the SDRAM 200.

The SDRAM 200 further includes an address register 206 operable to latchan address applied on an address bus 208, and output the latched addressto the control circuit 202, a column address latch 210, and a rowaddress multiplexer 212. During operation of the SDRAM 200, a rowaddress with a bank address BA and a column address with the bankaddress are sequentially latched by the address register 206 undercontrol of the control circuit 202. In response to the latched bankaddress BA and row address, the control circuit 202 controls the rowaddress multiplexer 212 to latch and output the row address to one of arow address latch 214 and 216. The row address latches 214 and 216, whenactivated, latch the row address from the row address multiplexer 212and output this latched row address to an associated row decoder circuit222 and 224, respectively. The row decoder circuits 222 and 224 decodethe latched row address and activate a corresponding row of memory cellsin memory banks 218 and 220, respectively. The memory banks 218 and 220each include a number of memory cells (not shown) arranged in rows andcolumns, each memory cell operable to store a bit of data and having anassociated row and column address.

When a column address and bank address BA is applied on the address bus208, the column address is latched by the address register 206 undercontrol of the control circuit 202, and output to a column address latch210, which latches the column address and in turn outputs the columnaddress to a burst counter circuit 226. The burst counter circuit 226operates to develop sequential column addresses beginning with thelatched column address when the SDRAM 200 is operating in a burst mode.The burst counter 226 outputs the developed column addresses to a columnaddress buffer 228, which in turn outputs the developed column addressto a pair column decoder circuits 230 and 231. The column decodercircuits 230 and 231 decode the column address and activates one of aplurality of column select signals 232 corresponding to the decodedcolumn address. The column select signals 232 are output to senseamplifier and I/O gating circuits 234 and 236 associated with the memorybanks 218 and 220, respectively. The sense amplifier and I/O gatingcircuits 234 and 236 sense and store the data placed on the digit lines235 and 237, respectively, by the memory cells in the addressed row andto thereafter couple the digit lines 235 or 237 corresponding to theaddressed memory cell to an internal data bus 238. The internal data bus238 is coupled to a data bus 240 of the SDRAM 200 through either a datainput register 242 or a data output register 244. In the preferredembodiment, the adjustable time delay circuit 102 is coupled to the dataoutput register 244. This circuit is used to adjust the time data ispresented to the data bus in reference to the clock signal CLK. A datamask signal DQM controls the circuits 234 and 236 to avoid datacontention on the data bus 240 when, for example, a READ command isfollowed immediately by a WRITE command, as known in the art.

In operation, during a read data transfer operation, an externalcircuit, such as a processor, applies a bank address BA and row addresson the address bus 208 and provides an ACTIVE command to the commanddecode circuit 204. This applied address and command information islatched by the SDRAM 200 on the next rising edge of the clock signalCLK, and the control circuit 202 thereafter activates the addressedmemory bank 218 or 220. The supplied row address is coupled through therow address multiplexer 212 to the row address latch 214 or 216associated with the addressed bank, and this row address is thereafterdecoded and the row of memory cells in the activated memory bank 218 or220 is activated. The sense amplifiers in the sense amplifier and I/Ogating circuit 234 or 236 sense and store the data contained in eachmemory cell in the activated row of the addressed memory bank 218 or220.

The external circuit thereafter applies a READ command to the commanddecode circuit 204 including a column address and bank address BA on theaddress bus 208, both of which are latched on the next rising edge ofthe clock signal CLK. The latched column address is then routed throughthe circuits 210, 226, and 228 to the column decoder circuit 230 undercontrol of the control circuit 204. The column decoder 230 decodes thelatched column address and activates the column select signal 232corresponding to that decoded column address. In response to theactivated column select signal 232, the sense amplifier and I/O gatingcircuit 234 or 236 transfers the addressed data onto the internal databus 238, and the data is then transferred from the internal data bus 238through the data output register 244 and onto the data bus 240 where itis read by the external circuit.

During a write data transfer operation, after activating the addressedmemory bank 218 or 220 and the addressed row within that bank, theexternal circuit applies a WRITE command to the command decode circuit204 including a column address and bank address BA on the address bus208 and data on the data bus 240. The WRITE command, column address, anddata are latched respectively into the command decode circuit 204,address register 206 and data input register 242 on the next rising edgeof the clock signal CLK or an internal clock signal not generated by thedelay adjusting circuit 60. The data latched in the data input register242 is placed on the internal data bus 238, and the latched columnaddress is routed through the circuits 210, 226, and 228 to the columndecoder circuit 230 under control of the control circuit 204. The columndecoder 230 decodes the latched column address and activates the columnselect signal 232 corresponding to that decoded address. In response tothe activated column select signal 232, the data on the internal databus 238 is transferred through the sense amplifier and I/O gatingcircuit 234 or 236 to the digit lines 235 or 237 corresponding to theaddressed memory cell. The row containing the addressed memory cell isthereafter deactivated to store the written data in the addressed memorycell.

Although the adjustable time delay circuit 102 has been described asbeing used in the SDRAM 200, it will be understood that it may also beused in other types of integrated circuits such as synchronous graphicsRAM (SGRAM), or synchronous static RAM (synchronous SRAM). Those skilledin the art realize the differences between SDRAM and other types ofmemories, and can easily implement the adjustable time delay circuit102.

FIG. 7 is a block diagram of a computer system 300 including the SDRAM200 of FIG. 5. The computer system 300 includes a processor 302 forperforming various computing functions, such as executing specificsoftware to perform specific calculations or tasks. Coupled to theprocessor 302 is a synchronous SRAM circuit 303, used for a memory cacheor other memory functions. In addition, the computer system 300 includesone or more input devices 304, such as a keyboard or a mouse, coupled tothe processor 302 to allow an operator to interface with the computersystem 300. Typically, the computer system 300 also includes one or moreoutput devices 306 coupled to the processor 302, such output devicestypically being a printer or a video terminal. One or more data storagedevices 308 are also typically coupled to the processor 302 to storedata or retrieve data from external storage media (not shown). Examplesof typical data storage devices 308 include hard and floppy disks, tapecassettes, compact disk read-only memories (CD-ROMs), and digitalvideodisk read-only memories (DVD-ROMs). The processor 302 is typicallycoupled to the SDRAM 200 and to the synchronous SRAM 303 through acontrol bus, a data bus, and an address bus to provide for writing datato and reading data from the SDRAM and synchronous SRAM. A clockingcircuit (not shown) typically develops a clock signal driving theprocessor 302, SDRAM 200, and synchronous SRAM 303 during such datatransfers.

It is to be understood that even though various embodiments andadvantages of the present invention have been set forth in the foregoingdescription, the above disclosure is illustrative only, and changes maybe made in detail, and yet remain within the broad principles of theinvention. Therefore, the present invention is to be limited only by theappended claims.

What is claimed is:
 1. A data delay circuit comprising: an externalclock terminal adapted to receive an external clock signal having afirst state and a second state; an adjustable time delay circuit havinga plurality of control inputs each adapted to receive a respectivecontrol signal, the time delay circuit adapted to receive the externalclock signal at a clock input and pass a delayed clock signal to anoutput terminal, the time between when the external clock signal changesstates and when the delayed clock signal changes states determined bythe control signals received at the control inputs, the adjustable timedelay circuit including a plurality of passgates coupled in series withrespective delay elements, each of the passgates being selectivelycontrolled by at least one of the control signals to control the timebetween when the external clock signal changes states and when thedelayed clock signal changes states; and a data passing circuitreceiving both the data signal and the delayed clock signal, the datapassing circuit adapted to pass the data signal from the data terminalto a data output terminal after the delayed clock signal changes fromthe first to the second state.
 2. The circuit of claim 1 wherein thecontrol signals are generated by selectively coupling the control inputsto a reference voltage.
 3. The circuit of claim 2 wherein each of thecontrol inputs is selectively coupled to a reference voltage using arespective fuse.
 4. The circuit of claim 2 wherein each of the controlinputs is selectively coupled to a reference voltage using a respectiveantifuse.
 5. The circuit of claim 1 wherein the: plurality of passgatesare arranged in columns, each column having a different number ofpassgates than any other column in the time delay circuit, the passgatesin the column having the highest number of passgates in the delaycircuit each coupled to a respective one of the delay elements, and theexternal clock signal coupled to and passing through each of the delayelements.
 6. The circuit of claim 5 wherein one of the delay elementshas a null delay value.
 7. The circuit of claim 5 wherein each of thecolumns is coupled to the columns adjacent to it such that one-half ofthe total number of passgates in each column are coupled to one-half ofthe total number of passgates in each adjacent column.
 8. The circuit ofclaim 5 wherein the number of control inputs to the delay circuit is n,the number of columns in the time delay circuit is n, and the number ofpassgates in the column having the highest number of passgates in thedelay circuit is 2^(n).
 9. The circuit of claim 5 wherein each controlsignals controls all of the passgates in a respective one of thecolumns, with one-half of the passgates in the respective columncontrolled to a passing state and the remainder of the passgates in therespective column controlled to a blocking state responsive to therespective control signal.
 10. A clocked-data phase adjustment circuitcomprising: a reference clock terminal adapted to receive a referenceclock signal having a first state and a second state; a data terminaladapted to receive a data signal; and a phase shifting circuit adaptedto receive a plurality of control signals, the reference clock signal,and the data signal each at respective inputs, the phase shiftingcircuit being adapted to adjust the time when the data signal is passedfrom the data input terminal to a data output terminal responsive to thephase of the reference clock signal, the phase of the reference clocksignal being determined by the control signals, the phase shiftingcircuit including a plurality of passgates coupled in series withrespective delay elements, each of the passgates being selectivelycontrolled by at least one of the control signals to control the phaseof the reference clock signal.
 11. The circuit of claim 10 wherein thecontrol signals are generated by selectively coupling the control inputsto a reference voltage.
 12. The circuit of claim 11 wherein each of thecontrol inputs is selectively coupled to a reference voltage using arespective fuse.
 13. The circuit of claim 11 wherein each of the controlinputs is selectively coupled to a reference voltage using a respectiveantifuse.
 14. The circuit of claim 10 wherein the: passgates arearranged in columns, each column having a different number of passgatesthan any other column in the phase shifting circuit, the passgates inthe column having the highest number of passgates in the delay circuiteach coupled to a respective one of the delay elements, and the externalclock signal being coupled to and passing through each of the delayelements.
 15. The circuit of claim 14 wherein one of the delay elementshas a null delay value.
 16. The circuit of claim 14 wherein each of thecolumns is coupled to the columns adjacent to it such that one-half ofthe total number of passgates in each column are coupled to one-half ofthe total number of passgates in each adjacent column.
 17. The circuitof claim 14 wherein the number of control inputs to the delay circuit isn, the number of columns in the time delay circuit is n, and the numberof passgates in the column having the highest number of passgates in thedelay circuit is 2^(n).
 18. The circuit of claim 14 wherein each controlsignals controls all of the passgates in a respective one of thecolumns, with one-half of the passgates in the respective column in apassing state and the remainder of the passgates in the respectivecolumn in a blocking state responsive to the respective control signal.19. In a synchronous memory circuit, a circuit to adjust output datatiming comprising: an external clock terminal adapted to receive anexternal clock signal having a first state and a second state; a clockfiltering circuit coupled to the external clock terminal, the clockfiltering circuit adapted to modifying the external clock signal into adata clock signal; a data terminal adapted to receive a data signal; anadjustable time delay circuit having a plurality of control inputs eachreceiving a respective control signal, the time delay circuit adapted toreceive the data clock signal at a clock input and passing a timeadjusted data clock signal to an output terminal, the time between whenthe data clock signal changes states and when the time adjusted dataclock signal changes states determined by the control signals receivedat the control inputs, the adjustable time delay circuit including aplurality of passgates coupled in series with respective delay elements,each of the passgates being selectively controlled by at least one ofthe control signals to control the phase of the time adjusted data clocksignal relative to the phase of the data clock signal; and a datapassing circuit adapted to receive both the data signal and the timeadjusted data clock signal, the data passing circuit adapted to pass thedata signal from the data terminal to a data output terminal after thetime adjusted data clock signal changes from the first to the secondstate.
 20. The circuit of claim 19 wherein the control signals aregenerated by selectively coupling the control inputs to a referencevoltage.
 21. The circuit of claim 19 wherein the: passgates are arrangedin columns, each column having a different number of passgates than anyother column in the time delay circuit, the passgates in the columnhaving the highest number of passgates in the delay circuit each coupledto a respective one of the delay elements, the external clock signalcoupled to and passing through each of the delay elements.
 22. Thecircuit of claim 21 wherein the number of control inputs to the delaycircuit is n, the number of columns in the time delay circuit is n, andthe number of passgates in the column having the highest number ofpassgates in the delay circuit is 2^(n).
 23. The circuit of claim 19wherein the synchronous memory circuit is a synchronous static randomaccess memory.
 24. A method of adjusting the time that data is deliveredto an output terminal of a memory circuit after the memory circuit hasbeen fabricated, the method comprising the steps of: accepting a datasignal at a data input; accepting a clock signal at a clock input;selecting a delay time to be added to the clock signal to generate adelayed clock signal, the delay time being selected by selectivelycoupling control inputs to a control voltage, the control inputs coupledto and controlling a plurality of passgates such that for any singlepattern of coupled control inputs, there exists only one path through acontrollable delay circuit; and using the delayed clock signal tocontrol the time when the data signal is passed to a data output. 25.The method of claim 24 wherein the delay time selected is one of aplurality of pre-selected delay times.
 26. The method of claim 24wherein the step of selecting a delay time further includes the step ofselectively coupling control inputs to a control voltage, the controlinputs coupled to and controlling a plurality of the passgates such thatfor any single pattern of coupled control inputs, there exists only onepath through a controllable delay circuit.